@Autowired for dependency injection in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What does @Autowired do in Spring Boot?

In Spring Boot, what is the primary purpose of the @Autowired annotation?

A It marks a class as a Spring Boot application entry point.

B It automatically injects the required dependency into a class field or constructor.

C It defines a REST endpoint in a controller class.

D It configures database connection properties.

💡 Hint

Think about how Spring Boot provides objects to classes without manual creation.

@Autowired tells Spring Boot to provide the needed object automatically, so you don't have to create it yourself.

❓ component_behavior

intermediate

What happens if Spring cannot find a bean to @Autowired?

Consider a Spring Boot class with a field annotated with @Autowired. What happens if Spring cannot find a matching bean to inject?

A Spring creates a new bean automatically without any error.

B Spring logs a warning but continues running.

C The field remains null and the application runs without errors.

D The application fails to start with a NoSuchBeanDefinitionException.

💡 Hint

Think about what Spring expects when it cannot find a required dependency.

If Spring cannot find a bean to inject, it throws an exception and stops the application startup to avoid null references.

📝 Syntax

advanced

Which code snippet correctly uses @Autowired for constructor injection?

Select the code snippet that correctly uses @Autowired to inject a dependency via constructor in a Spring Boot component.

A

public class Service {
  private final Repo repo;
  @Autowired
  public Service(Repo repo) {
    this.repo = repo;
  }
}

B

public class Service {
  @Autowired
  private Repo repo;
  public Service() {}
}

C

public class Service {
  private Repo repo;
  public Service(@Autowired Repo repo) {
    this.repo = repo;
  }
}

D

public class Service {
  private Repo repo;
  public Service(Repo repo) {
    this.repo = repo;
  }
}

💡 Hint

Look for the correct placement of @Autowired on the constructor.

The correct way is to put @Autowired on the constructor so Spring knows to inject the dependency when creating the object.

🔧 Debug

advanced

Why does this @Autowired field remain null?

Given this Spring Boot component code, why does the service field remain null? public class Controller { @Autowired private Service service; public void doWork() { service.perform(); } } Assume Service is a valid Spring bean.

A The Controller class is not managed by Spring (missing @Component or similar).

B The Service bean is not annotated with @Service.

C The @Autowired annotation is missing on the service field.

D The perform() method does not exist in Service.

💡 Hint

Think about which classes Spring manages and injects dependencies into.

If the Controller is not a Spring-managed bean, Spring will not inject dependencies into it, so the field stays null.

❓ lifecycle

expert

When is the @Autowired injection performed in Spring Boot lifecycle?

At what point in the Spring Boot application lifecycle does the @Autowired injection happen for a bean?

A After the application context is fully started and running.

B Before the constructor is called, during class loading.

C During bean creation, after the constructor but before any @PostConstruct methods.

D Only when the bean is first accessed by another bean.

💡 Hint

Consider when Spring sets dependencies relative to constructor and lifecycle callbacks.

Spring injects dependencies right after creating the bean instance (constructor call) but before calling any @PostConstruct methods.